Radio-imaging of underground structures

ABSTRACT

A coal bed anomaly detection and imaging system comprises a synchronous transmitter and receiver that are separated by a geologic structure with embedded and hidden anomalies. The transmitter sends out two signals from magnetic dipole antennas. Such signals are widely separated in frequency but synchronized internally in the transmitter to one another. The higher frequency is used to make phase shift and attenuation measurements at the receiver by synchronous detection. The lower frequency is used at the receiver to synchronize the receiver to the transmitter. The higher frequency signal is measurably affected by anomalies in the intervening geologic structure. The lower frequency signal is fixed low enough so it is not substantially affected by the intervening geologic structure. Geologic modeling tools are preferably downloaded by geoscientists to their personal computers. The total attenuation and phase shift measurements are plugged into a two-dimensional and three-dimensional full-wave inversion code (FWIC) process. A hypothetical model is uploaded for processing by a forward solver so the nature of the anomalous geologic structure can be estimated. A resulting reconstructed image of the anomalies in silhouette is then downloaded for interpretation of the image by the geoscientist.

COPENDING APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication, Ser. No. 10/047,073, filed Jan. 15, 2002, and titled METHODAND SYSTEM FOR RADIO-IMAGING UNDERGROUND GEOLOGIC STRUCTURES. Such isincorporated herein by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates to ground-penetrating radars andcoal mining, and more particularly to methods and systems forradio-imaging anomalous geology in coal bed deposits.

[0004] 2. Description of the Prior Art

[0005] Given the growing need to produce cleaner run-of-mine (ROM) coal,improved information about the seam geology and coal quality in coalmine operations is of great value. The identification of anomalies isimportant so planning operations keep productivity high and cut cleanercoal. For example, the identification of a paleochannel anomaly beforemining began would allow longwall panels to be laid out to avoidcrossing it.

[0006] A natural coal seam waveguide occurs in layered sedimentarygeology because the electrical conductivity of the bounding shale,mudstone, and fire clay, ranges between 0.01 and 0.1 Siemens per meter(S/m) (100 and 10 ohm-meters). Inside, the conductivity of the coal isnear 0.0005 S/m (2,000 ohm-meters). The 10:1 conductivity contrastenables the waveguide travel of electromagnetic waves within the coalbed.

[0007] The electric field (E_(z)) component of a travelingelectromagnetic wave (EM) is polarized in a vertical direction and themagnetic field (H_(y)) component is polarized horizontally in the seam.The energy in this part of the EM wave travels in the coal seam from thetransmitter to the radio imaging receiver. There is a horizontallypolarized electric field (E_(x)) that has zero value in the center ofthe seam and reaches maximum value at the sedimentary rock-coalinterface. This component is responsible for transmission of theelectromagnetic wave signal into the boundary rock layer. The energy inthis part of the EM wave travels vertically in the coal deposit.

[0008] The magnitude of coal seam radiowave decreases as it travelsalong the waveguide. The attenuation rate and cylindrical spreading ofwave energy in the coal seam are two of the things at work thatattenuate the traveling signals. The cylindrical spreading factor is$\frac{1}{\sqrt{r}},$

[0009] where r is the distance from the transmitting to receivingantenna. This factor compares with the non-waveguide far-fieldspherically spreading factor of $\frac{1}{r}.$

[0010] Thus, for a given separation of one-hundred meters, the magnitudeof the seam EM wave decreases by ten in a waveguide, and by a factor ofone-hundred in an unbounded media. So one advantage of sending signalsdown a seam waveguide is the much greater travel distance. Anotheradvantage is that the traveling electromagnetic wave predominantly stayswithin the coal seam, the main item of interest.

[0011] A coal-seam electromagnetic wave is very sensitive to changes inthe waveguide geometry and materials. The radiowave attenuation rate(decibels per 100 feet) and phase shift (electrical degrees per 100feet) were determined by Dr. David Hill at the National Institute forScience and Technology (NIST). Dr. James Wait was the first to recognizethat natural waveguides exist in the earth's crust. Both are Fellows inthe Institute of Electrical and Electronic Engineers. The scienceunderlying the traveling of an electromagnetic wave in the coal seamwaveguide is well known. The engineering of both the crossholetransmitter and receiver has also been developed to a high degree ofperformance. The transmitter and receiver are synchronized to enable themeasurement of total path phase shift from the transmitter to thereceiver location. The total phase shift measurement is a distinguishingfactor in the radio imaging-IV instrumentation. Prior art radio imaginginstrumentation measures only the change in magnitude of radiowave,e.g., attenuation, when propagating in the coal seam waveguide.

[0012] In uniform-construction waveguides, the path is a straight line.The path length or distance a radio signal travels can be determinedfrom phase-shift measurements. The straight line path is an assumptionused in the Algebraic Reconstruction Technique (ART) tomographyalgorithm. But radiowaves are refracted near significant geologicanomalies causing the travel path of the radiowave to bend and be longerthan in the uniform waveguide case. This bending cannot be accounted forin ART processing and accounts for this distortion in the ART tomographyprocessing algorithm. By measuring the total path phase shift, thebending effect can be accounted for in a new type of tomographyreconstruction algorithm called Full-wave Inversion Code (FWIC). SuchFWIC can reconstrue three-dimensional images of anomalous geology. Radioimaging IV (RIM-4) instrumentation acquires data that can be processedin the Sandia National Laboratories' WAIC algorithm. The effect ofattenuation in the waveguide is to reduce the magnitude of theelectromagnetic wave along the path.

[0013] Under sandstone sedimentary rock, the attenuation rate increasesbecause more of the radio imaging signal travels vertically into theboundary rock, i.e., leaks from the waveguide. If water is injected intothe coal, then clay in the coal causes the electrical conductivity todecrease and the attenuation rate/phase shift to increase.

[0014] The attenuation rate/phase shift rapidly increases withdecreasing seam height. Thus coal seam thinning can be easily detectedwith radio imaging. The above graphical presentation of coal seamwaveguide attenuation and phase constants represents the science factorin the art and science of interpreting radio imaging tomographic images.Higher attenuation rate zones suggest that either the coal seam boundaryrock is changing, the seam is rapidly thinning, or/and water has beeninjected into the coal seam. Drilling and radar would determine theexact cause of the anomalous seam condition. This advance in the stateof the art in mining would reduce both risk and cost in coal extraction.

[0015] Faults and dykes cause reflections to occur in the waveguide. Thereflections can appear as excess path loss. Total phase shiftmeasurements are useful in detecting reflection anomalies.

[0016] The predominating electromagnetic wave propagation mode in layersof coal is a “seam wave”. Such is polarized in the vertical plane of theseam, and has a uniform, polarized electric field orthogonal to thelayer. In horizontal lying coal bed layers, the magnetic field will behorizontally polarized with the same field strength across a verticalcross-section. The electric field is vertically polarized. A thirdelectric field is polarized in the horizontal plane and is maximum valueat each boundary of the seam.

[0017] The horizontal component of the electric field is null near thephysical center of the coal seam, albeit if the lower-resistivityboundary layers above and below are about equal in their respectivematerial electrical resistivities.

[0018] The present inventor, Larry G. Stolarczyk, has described methodsand equipment for imaging coal formations in geologic structures in manyUnited States patents. Some of those patents are listed in Table I, andare incorporated herein by reference. TABLE I Patent No. Issued TitleUS04577153 Mar. 18, 1996 Continuous Wave Medium Frequency SignalTransmission Survey Procedure For Imaging Structure In Coal SeamsUS04691166 Sep. 1, 1987 Electromagnetic Instruments For ImagingStructure In Geologic Formations US04742305 May 3, 1988 Method ForConstructing Vertical Images Of Anomalies In Geological FormationsUS04753484 Jun. 28, 1988 Method For Remote Control Of A Coal ShearerUS04777652 Oct. 11, 1988 Radio Communication Systems For UndergroundMines US04879755 Nov. 7, 1989 Medium Frequency Mine Communication SystemUS04968978 Nov. 6, 1990 Long Range Multiple Point Wireless Control AndMonitoring System US04994747 Feb. 19, 1991 Method And Apparatus ForDetecting Underground Electrically Conductive Objects US05066917 Nov.19, 1991 Long Feature Vertical Or Horizontal Electrical ConductorDetection Methodology Using Phase Coherent ElectromagneticInstrumentation US05072172 Dec. 10, 1991 Method And Apparatus ForMeasuring The Thickness Of A Layer Of Geologic Material Using AMicrostrip Antenna US05087099 Feb. 11, 1992 Long Range Multiple PointWireless Control And Monitoring System US05093929 Mar. 3, 1992 MediumFrequency Mine Communica- tion System US05121971 Jun. 16, 1992 Method OfMeasuring Uncut Coal Rib Thickness In A Mine US05146611 Sep. 8, 1992Mine Communications Cable And Method For Use US05181934 Jan. 26, 1993Method For Automatically Adjusting The Cutting Drum Position Of AResource Cutting Machine US05188426 Feb. 23, 1993 Method For ControllingThe Thickness Of A Layer Of Material In A Seam US05260660 Nov. 9, 1993Method For Calibrating A Downhole Receiver Used In ElectromagneticInstrumentation For Detecting An Underground Conductor US05268683 Dec.7, 1993 Method Of Transmitting Data From A Drillhead US05301082 Apr. 5,1994 Current Limiter Circuit US05408182 Apr. 18, 1995 Facility And MehodFor The Detection And Monitoring Of Plumes Below A Waste ContainmentSite With Radio- wave Tomography Scattering Methods US05474261 Dec. 12,1995 Ice Detection Apparatus For Transportation Safety US05686841 Nov.11, 1997 Apparatus And Method For The Detec- tion And Measurement OfLiquid Water And Ice Layers On The Surfaces Of Solid MaterialsUS05769503 Jun. 23, 1998 Method And Apparatus For A Rotating CuttingDrum Or Arm Mounted With Paired Opposite Circular Polarity Antennas AndResonant Microstrip Patch Transceiver For Measuring Coal, Trona AndPotash Layers Forward, Side And Around A Con- tinuous Mining MachineUSRE032563 Dec. 15, 1987 Continuous Wave Medium Frequency SignalTransmission Survey Procedure For Imaging Structure In Coal SeamsUSRE033458 Nov. 27, 1990 Method For Constructing Vertical Images OfAnomalies In Geological Formations

[0019] In underground coal mining practice, horizontal magnetic dipoleantennas can be driven by a radio transmitter so a seam wave willpropagate within the coal, or other layer of higher-resistivity media. Aremote, horizontal magnetic dipole receiving antenna is then used tomeasure the seam wave with a receiver synchronized to the transmitter.Fiber-optic cables are preferably used for the receiver-transmittersynchronization, e.g., because a metallic cables would interfere withreception by receiving the transmitted signals, and re-radiating them tocompete with the direct signal to the receiver. Phase coherent receiverdesign allows synchronous detection and accurate phase measurements ofthe direct signal. The effects on direct signal phase help elicit thenature of the coal layer, given a priori or concomitant materialdielectric-constant measurements.

[0020] However, the logistics of providing the synchronization cable canprove impossible in some mines and in some applications. So it would bedesirable to synchronize such transmitters and receivers withoutrequiring a cable between the receiver and transmitter.

SUMMARY OF THE INVENTION

[0021] It is therefore an object of the present invention to provide aninstrument for the detection and high-resolution imaging of anomalousgeologic structures.

[0022] It is another object of the present invention to provide animaging method for simultaneously transmitting a synchronizing and animaging electromagnetic wave from transmitting locations to receiverlocations for phase coherent detection by an imaging receiver.

[0023] It is still another objective of the present invention to providea method of receiver phase coherency with a transmitter and therebyobtain maximum receiver threshold sensitivity when measuring totalattenuation and phase shift of imaging electromagnetic waves passingthrough a geologic target.

[0024] It is still another object of the present invention to provide afull-wave inversion code method in the reconstruction of imaging ofanomalous geology when the ray path assumption in the algebraicreconstruction algorithm becomes invalid.

[0025] Briefly, a coal bed anomaly detection and imaging embodiment ofthe present invention comprises a synchronous transmitter and receiverthat are separated by a geologic structure with embedded and hiddenanomalies. Two transmitters send out two signals from twomagnetic-dipole antennas. Such signals are widely separated in frequencybut synchronized together. The RIM imaging transmitter transmits ahigher frequency which is used to make phase shift and attenuationmeasurements at the RIM receiver by synchronous detection. Thesynchronizing transmitter transmits a lower frequency sync-signal and isused at the synchronizing receiver to recover the synchronization. Therecovered sync-signal is sent by fiberoptic cable to the RIM receiver.Both the sync-transmitter and sync-receiver stay put during a survey inwhich the RIM transmitter and receiver are moved about. The highfrequency RIM signal is measurably affected by anomalies in theintervening geologic structure. The lower frequency signal is fixed lowenough so it is not substantially affected by the intervening geologicstructure. Geologic modeling tools are preferably downloaded bygeoscientists to their personal computers. The total attenuation andphase shift measurements are e-mailed to a data processing center wherethey are plugged into a full-wave inversion code (FWIC) process. Ahypothetical model is uploaded for processing by a forward solver so thenature of the anomalous geologic structure can be estimated by thegeoscientist at the mine or an interpretation center. A resultingreconstructed image of the anomalies in silhouette is then downloadedfor interpretation of the image by the geoscientist.

[0026] An advantage of the present invention is a synchronized system isprovided that eliminates the need for a synchronization cable wiredbetween a transmitter and a receiver.

[0027] Another advantage of the present invention is a method isprovided for the measurement of the total attenuation and phase shift ofan imaging electromagnetic wave after its having propagated through ageologic target.

[0028] A further advantage of the present invention is that the measureddata can be processed in an image reconstruction code. Such method isnot invalidated by refraction, reflection, and scattering of an imagingelectromagnetic wave in a geologic target.

[0029] These and other objects and advantages of the present inventionwill no doubt become obvious to those of ordinary skill in the art afterhaving read the following detailed description of the preferredembodiments which are illustrated in the various drawing figures.

IN THE DRAWINGS

[0030]FIG. 1 is a plan-view diagram of an underground, longwall coalmining operation that is being imaged between roadways for anomalies bya radio probe system embodiment of the present invention;

[0031]FIG. 2 is a plan-view diagram of another underground, longwallcoal mining operation that is being imaged between boreholes in thelongwall panel for anomalies by a drillstring radio probe systemembodiment of the present invention; and

[0032]FIG. 3 is a function block diagram of a radio probe systemembodiment of the present invention useful in the systems shown in FIGS.1 and 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0033]FIG. 1 represents an underground, longwall coal mining operation100 in plan view. A main development roadway 102 has several branchinglongwall roadways, e.g., roadways 104 and 106. These roadways are cutbetween a number of coal bed longwall panels 108, 110, 112, and 114.Conveyer belts, railroads, and trains are typically used in the maindevelopment roadway 102 to haul-out the coal being mined. The typical,modern longwall coal mining operation 100 uses the so-calledroom-and-pillar method where large pillars of coal are left to supportthe roof and thereby prevent surface subsidence. The parts of the roofbetween the pillars are typically reinforced with six-foot long roofbolts which attach metal screens as part of the roof control in theapproved mining plan. The roadways and panels between them can stretchseveral thousand feet in length. The panels are typically eight hundredfeet wide. Conveyor belts and shuttle cars are used to move the coalfrom the cut faces to the train cars or conveyer belts.

[0034] The coal bed longwall panels 108, 110, 112, and 114, can, andusually do include anomalies, that are best left uncut. Such anomaliescan comprise faults, dikes, water, sandstone, limestone, and othermaterials lacking coal and/or presenting potential mining hazards. Forexample, mining into an underground paleochannel, abandoned mine, orfault, can cause the mine to flood. Flooding can require expensivepumping and long delays in getting the water cleared out.

[0035] In the past, using conventional methods and equipment, theseanomalies were not discovered until actually encountered. Knowing wherethese anomalies lie early in the mine development, can allow betterplanning of the overall mining operation, and allow the optimumroom-and-pillar geometries and placements. Some traditional methodsexist for developing a rough idea of the size, nature, and orientationof anomalies, but the electronic systems and methods described hereinallow much better pictures and assessments to be had.

[0036] Embodiments of the present invention permit the longwall panelsto be electronically imaged by passing radio waves through them. Inparticular, the ways these radio waves are affected can be interpretedas showing anomalies in the otherwise homogeneous coal deposits. Theanomalies will impart a different phase shift and attenuation on theradio waves than will coal. But such measurements requiresynchronous-detection by the receiver.

[0037] Therefore, an anomaly-imaging system embodiment of the presentinvention includes a radio-imaging (RIM) transmitter 116 with atransmitting antenna 118 that beams out a radio signal 120. The longwallpanel 110 with its waveguide characteristic, for example, will impart aphase shift and attenuation to the radio signal that will be observed bya receiving antenna 122 and a RIM-receiver 124. A synchronizing,wireless signal 126 is sent by synchronizer transmitter 128 and its loopantenna 129 to synchronizing receiver 130 and its loop antenna 131. Thefrequency of wireless synchronizing signal 126 is preferably in therange of 20-KHz. So the attenuation and total phase shift it experiencespassing through longwall panel 110 will be relatively small. Afiberoptic cable 132 provides a synchronizing clock from RIM-transmitter116 to synchronizer transmitter 128. Similarly, a fiberoptic cable 134provides a recovered synchronizing clock from synchronizer receiver 130to RIM-receiver 124.

[0038] In order to collect tomographic data so an image of the anomaliescan be reconstructed, the RIM-transmitter 116, its antenna 118, theRIM-receiver 124 and its antenna 122 are respectively moved up and downroadways 104 and 106. The synchronizer transmitter 128, loop antenna129, synchronizing receiver 130, and loop antenna 131 are typically notmoved during a survey.

[0039] Signal 120 passes through the longwall panel 110 at may differentperspectives during these repositionings of the RIM transmitter andreceiver, and the phase and attenuation experienced with eachRIM-transmitter and RIM-receiver location is logged into a database. Theimages of the hidden anomalies can be tomographically processed from thedatabase in real-time or in post processing.

[0040] The construction of the RIM-transmitter 116 and the RIM-receiver124, as well as the methods used to reconstruct tomographic images ofunderground anomalies and of coal deposits are suggested and describedin many of the present inventor's previously issued patents. Forexample, the reader is referred to United States patents: U.S. Pat. Nos.4,691,166, issued Sep. 1, 1987; 4,577,153, issued Mar. 18, 1986; Re-Exam32,563, issued Dec. 15, 1987; 4,742,305, issued May 3, 1988; 5,408,182,issued Apr. 18, 1995, and Re-Exam 33,458, issued Nov. 27, 1990. All suchpatents are incorporated herein by reference.

[0041] In embodiments of the present invention, it is assumed to beimpractical to string a cable between synchronizers 128 and 130. So itis a critical part of the present invention that the communicationbetween synchronizers 128 and 130 must be wireless. Power supply cordsand utility power sources can also be impractical, so battery operationis necessitated for the RIM-transmitter 116, RIM-receiver 124, andsynchronizers 128 and 130.

[0042] Synchronization enables the detection and measurement of thesmallest sinusoidal signal embedded in electrical noise. When thereceiver design is phase coherent with the synchronization signal andsynchronous detection is employed in the receiver, the attenuation rate(alpha) and phase shift (beta) of an electromagnetic wave propagatingbetween the RIM transmitting and receiving magnetic dipole antennas canbe measured. As used herein,

[0043] H is the magnitude of the magnetic field threading the area ofthe RIM-receiver magnetic dipole;

[0044]

=2

f and f is the frequency in Hertz;

[0045] μ_(o) is the magnetic permeability of free space;

[0046] A is the area in square meters of the magnetic dipole;

[0047] μ_(r) is the relative permeability of the material upon which thecoils of wire are wound; and

[0048] N is the number of turns used to wind the magnetic dipoleantenna.

[0049] Images of geologic structure are constructed by dividing thegeologic region into pixels or boxels included a plane or volume betweenlocations visited by the RIM receiver and transmitter. Lines between thelocations visited represent the edges of the image plane. The depth,width and height of each pixel or boxel is arbitrary, but is usuallymade unequal to the physical space between each transmitter and receiverlocation. One method of determining the electromagnetic wave propagationconstants in each pixel is the well-known Algebraic ReconstructionTechnique (ART) described in U.S. Pat. No. 5,260,660, issued Nov. 9,1993.

[0050] The ART method assumes an electromagnetic wave propagation willfollow a straight line path between the transmitter and receiverlocations. Such assumption is reasonably valid in a uniform geologicsetting with anomalies. Anomalous geology can be mapped with distortionin the image aligned along the majority of ray paths. As the degree ofgeologic disturbance increases, the electromagnetic wave phenomena ofrefraction, reflection, and scattering increases and invalidates the raypath assumption in ART. Images reconstructed with the invalid assumptionexhibit artifacts, e.g., false images, in the image plane.

[0051] Clearly, it would be desirable to have a synchronizedelectromagnetic wave instrumentation that would not be logisticallyconstrained by requiring deployment of a synchronizing cable. Further,it would be desirable to have an image reconstruction algorithm thatwould not require a straight ray path assumption and that could accountfor electromagnetic wave propagation phenomena of refraction,reflection, and scattering in the geologic target.

[0052] The need for a synchronization cable is eliminated bysimultaneously transmitting a low-frequency electromagnetic wave 126 andthe imaging-frequency signal 120. Because attenuation increases withfrequency, the lower frequency, synchronization signal will propagatefurther. The carrier frequency of the synchronization signal 126 ispreferably in the range of 500-20,000 Hertz. The electrical conductivity(σ), dielectric constant (ε), and magnetic permeability (μ) all have animpact on the attenuation rate (α) and phase constant (β). Theseso-called quasi-static conditions and the synchronization signalconstant can be readily determined.

[0053] The synchronization receiver produces an output sync signal,which establishes the image signal receiver condition of phase coherencywith the transmit signal. The phase of the received signal relative tothe transmit signal can be measured. The instrumentation can measure thetotal phase shift in the electromagnetic wave when propagating from theRIM transmitter to RIM receiver location. Concurrently, the totalattenuation of the imaging signal can be measured. The magnitude andtotal phase shift of the electromagnetic wave can be measured.

[0054] The total field measurement enables the use of the full-waveinversion code (FWIC), a newly developed image forming algorithm. TheFWIC is defined as E_(T)=E_(i)+E_(s)=E_(b)+∫σE₁G(r₁r¹)dr, where E_(T) isthe total electric field measured by the RIM-receiver 124, E_(i) is theincident electric field calculated at a point in the longwall panel 110,

is the electrical conductivity distribution within the longwall panel110, G(r₁r¹) is the Green's function which accounts for wave propagationof the electromagnetic wave phenomena generated by an anomalous geologicstructure.

[0055] The FWIC requires a forward modeling algorithm to determine themagnitude and phase of the incident wave everywhere within the geologictarget. According to Maxwell's well-known equations, the incidentelectric field produces both conduction and displacement current flow inthe target. Conduction current predominates when σ/ωε>1, anddisplacement current predominates when σ/ωε<<1. By measuring the totalfield at each location, an iterative procedure is used to solve for theconductivity variation in the geologic target. The FWIC code does notrequire the straight ray path assumption and enables imagingreconstruction where anomalous geology causes wave propagation phenomenaof refraction, reflection, and scattering to occur within the geologictarget. See also, U.S. Pat. No. 5,408,182, issued Apr. 18, 1995.

[0056] The anomalous geology images will not rise tophotographic-quality pictures. For example, anomalous geologic objectscan appear in silhouettes in the image plane. Unknown geologic targetimages require interpretation by an expert, and involve art and science.

[0057]FIG. 2 represents another underground, longwall coal miningoperation 200, e.g., like that of FIG. 1 but in another stage ofdevelopment. As before, a main development roadway 202 has severalbranching longwall roadways, e.g., roadways 204 and 206. These roadwaysare cut between a number of coal bed longwall panels 208, 210, 212, and214. The coal bed longwall panels 208, 210, 212, and 214, can, andusually do include anomalies, e.g., anomaly 216. Such anomalies cancomprise faults, dikes, water, sandstone, limestone, and other materialslacking coal and/or presenting potential mining hazards.

[0058] Embodiments of the present invention permit the longwall panelsto be electronically imaged for anomalies by passing through a combinedhigh frequency probe radio wave and a low-frequency synchronizing radiowave. In FIG. 2, these signals are passed between de-gasificationboreholes 218 and 220.

[0059] Methane is conventionally degassed from the longwall panels byboring a series of horizontal bores spaced only a few hundred feetapart. For example, borehole 220 was drilled and degassed beforeborehole 218. A drillhead 222 is shown in the process of drillingborehole 218 and is fitted on its aft end with a transmitter 224. Adrillstring is not shown. A dual-frequency signal 226 is emitted bytransmitter 224 and is detected by a synchronous receiver 228. Thedual-frequency signal 226 includes a high-frequency probe component thatis substantially affected in signal strength and phase by anomaly 216.The dual-frequency signal 226 further includes a low-frequencysynchronizing component that is not substantially affected by anomaly216. The low-frequency synchronizing component is recovered in thereceiver 228 to synthesize a synchronous local oscillator, and suchenables synchronous detection of the high-frequency probe component.

[0060]FIG. 3 is an anomaly imaging system embodiment of the presentinvention, and is referred to herein by the general reference numeral300. The system 300 can be used effectively in the situations shown inFIGS. 1 and 2. The system 300 is used to image a coal-bed longwall panel302 for an anomaly 304. A probe transmitter 306 is positioned on oneside, and a probe receiver 308 is positioned on the other side. Ananomaly image processing sub-system 310 receives information about thereal physical locations of probe transmitter 306 and probe receiver 308.It further receives measurements of the signal strength and phase shiftsexperienced by an RF-probe signal 312. An RF-synchronizing signal 314 isused to convey transmitter synchronization information for synchronousdetection by the receiver 308. Such RF-synchronizing signal 314 has amuch lower carrier frequency than the RF-probe signal 312, e.g.,500-20,000 Hz. Because the signal frequency is in the range of500-20,000 Hz, the attenuation and total phase shift is small. The totalphase shift in the synchronization electromagnetic wave can thus beanalytically determined. The RF-probe signal 312 typically operates inthe range of 0.1-1.5 MHz.

[0061] A crystal 316 provides a reference frequency to an oscillator318. A modulator 320 accepts data that can be impressed on the RF-probesignal 312. A high-frequency radio power amplifier 322 builds up thesignal for emission by a high-frequency transmitter antenna 324, e.g., aferrite-core magnetic dipole type. A divider 326 provides a synchronouslow frequency for a low-frequency radio power amplifier 328. Theamplified RF-synchronizing signal 314 is output by a low-frequencytransmitter antenna 330, e.g., another ferrite-core magnetic dipoletype. The real physical position of the transmitter antenna 324 is sentby a telemetry transmitter 332 out through an antenna 334 on a signal336. Such transmitter information could alternatively be included in thedata being input at modulator 320, and the telemetry transmitter 332would not be necessary. The real physical position of the transmitterantenna 324 is needed in later tomographic processing to reconstruct animage of anomaly 304.

[0062] A low-frequency receiving antenna 338 captures theRF-synchronizing signal 314 for recovery of the carrier output bydivider 326. A low-frequency receiver 340 provides this signal forcomparison in phase and locking by a phase locked loop (PLL) 342. Thisdrives a voltage controlled oscillator (VCO) 344 to output the higherfrequency being input to divider 326 and transmitter as RF-probe signal312. A divider 346 helps accomplish this by having a divider 326.

[0063] The RF-probe signal 312 is received for measurement by ahigh-frequency antenna 348 and receiver 350. A synchronous detector 352detects the carrier signal to reveal any attenuation and phase shiftaffects caused by the anomaly 304. An in-phase (I) and quadrature-phase(Q) measurement is taken by a sampler 354 and forwarded to a datarecorder 356. The real physical position of receiving antenna 348 isalso recorded. In alternative embodiments, the position information oftransmitter antenna 324 may be recorded.

[0064] The receiver 308 can be shuttled or pushed along in a recentlycompleted de-gas hole. A companion transmitter may be built into ameasure-while-drilling (MWD) instrument and tomography scans areconducted during normal drilling, e.g., so as to not slow down coal bedmethane (CBM) drilling. The receiver in the prior drillhole ismaneuvered with the push rods or shuttle to acquire the tomographicdata. It is possible to line the hole with plastic pipe and use water orair pressure to maneuver the receiver during a tomographic scan.

[0065] On standing longwall panels, it is cost effective to first do across entry survey. The direct ray scan would acquire data at 50-ftintervals along the panel. The tomography scan would acquire diagonalpath data from each 50-ft measuring station. This would locate geologicdisturbance zones where CBM crosshole radio imaging is applied. Also,the radar is directed into these zones. In other words, radio imagingcrosshole would only be used in geologic disturbance zones.

[0066] The recorder stores information about the locations of RF-probeantennas 324 and 348, as well as the I,Q measurement information fromsampler 354. The locations of RF-probe antennas 324 and 348 indicate theray path RF-probe signal 312. The I,Q measurement information indicateswhether that ray path was affected and to what degree by the anomaly304. All this information is stored in a database 366 for real-time orpost processing. A full-wave inversion code (FWIC) process is usedtogether with an image reconstruction process 370 to build a displayimage for a user display 372. A silhouette 374 in a graphic image isused to represent the real anomaly 304.

[0067] For the most part, image interpreting geoscientists and otherusers have prerequisite skills and training in electromagnetic wavetheory. However, refraction, reflection, and scattering increase theuncertainties in the image because the assumption of a straight ray pathbecomes erroneous. Software tools can be downloaded (FTP) to thegeoscientist's personal computer, e.g., over the Internet. Using simplelayered models of hypothetical geology, the geoscientist can introducethe suspect geologic anomaly into the generic model and determine itselectromagnetic wave response using mathematical forward modeling code.The hypothetical model can be uploaded to a data processing center wherea forward modeling algorithm resides in an operating program. Theforward model output would be the total electromagnetic wave fieldsmeasurable at each receiver location.

[0068] These modeled data would also be processed, e.g., in the FWIC368, to form a hypothetical image. Also, the hypothetical image can bedownloaded to the geoscientist to improve scientific objectivity in thedata interpretation.

[0069] Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that thedisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the “true” spirit and scope of theinvention.

[0070] What is claimed is:

1. An anomaly sensing system, comprising: a transmitter providing for atleast one RF-probe signal and one RF-synchronizing signal atsubstantially different carrier frequencies and yet synchronous to oneanother; a coherent receiver having a frequency synthesizer able to lockon to said RF-synchronizing signal and providing for synchronousdetection of said RF-probe signal; and a measurement device fordetecting attenuation and phase shift affects on said RF-probe signal byan anomalous media that intervenes between the transmitter and receiver.2. The system of claim 1, wherein: the transmitter is disposed in afirst passageway in an underground coal mine or deposit; the receiver isdisposed in a second passageway in an underground coal mine or deposit;and the measurement device provides for measurements of any anomalieslying between said first and second passageways.
 3. The system of claim1, wherein: the transmitter includes a radio imaging (RIM) transmitterfor visiting a number of locations in a first passageway in anunderground coal mine or deposit; the receiver includes a radio imaging(RIM) receiver for visiting a number of locations in a second passagewayin an underground coal mine or deposit; and the measurement deviceprovides for tomographic measurements of anomalies intersected by raypaths between said locations along said first and second passageways. 4.The system of claim 1, further comprising: a drillstring and drillheadfor drilling boreholes in coal deposits, and which carries thetransmitter along a number of transmitting locations in a borehole in anunderground coal mine or deposit.
 5. The system of claim 1, wherein: thetransmitter is attached to a drillhead in a borehole for transmittingfrom a number of different locations an underground coal mine ordeposit.
 6. The system of claim 1, wherein: the receiver is disposed ina borehole and provides for reception of signals from the transmitter inanother borehole in an underground coal mine or deposit.
 7. The systemof claim 1, further comprising: means for repositioning the receiver ina borehole, and that provides for reception of signals from thetransmitter in another borehole in an underground coal mine or deposit.8. The system of claim 1, further comprising: a tomographic processorfor generating an image of any anomaly in said anomalous media that wasintersected by said RF-probe signal.
 9. The system of claim 1, wherein:the transmitter and receiver are such that said RF-probe signal is at acarrier frequency that is substantially affected in signal amplitude andphase by anomalies in said anomalous media that are intersected by saidRF-probe signal.
 10. The system of claim 1, wherein: the transmitter andreceiver are such that said RF-synchronizing signal is at a carrierfrequency that is not substantially affected in signal amplitude andphase by anomalies in said anomalous media.
 11. The system of claim 1,further comprising: a synchronous detector disposed in the receiver andable to use said RF-synchronizing signal to measure the strength andphase of said RF-probe signal.
 12. The system of claim 1, furthercomprising: a two-dimensional and three-dimensional tomographic imagereconstruction processor for employing full-wave inversion code (FWIC)to account for the affects of refraction, reflection, and bending ofsaid RF-probe signal between the transmitter and receiver.
 13. Thesystem of claim 1, further comprising: a two-dimensional andthree-dimensional tomographic image-reconstruction processor fordisplaying a graphic representation of an anomaly that lies hidden inthe ground between the transmitter and receiver.
 14. The system of claim1, further comprising: means for using simple three-dimensional layeredmodels of hypothetical geology and introducing a suspect geologicanomaly into a generic model to determine its electromagnetic waveresponse using mathematical forward modeling code in which the forwardmodel output is the total electromagnetic wave fields measurable at eachreceiver location.
 15. The system of claim 14, further comprising: meansfor processing modeled data with two-dimensional and three-dimensionalfull-wave inversion code (FWIC) to form a hypothetical image.